A growing body of evidence suggests that sub-concussive blows produce short- and long-term changes in brain function, and that there is a link between concussion and the severity of blows prior to the injury. In addition, groundbreaking research by scientists at Purdue[1,2] has found a strong correlation between the number and location of subconcussive impacts sustained by high school football players over the course of the season and deficits in neurocognitive performance and brain abnormalities, with the data suggesting that that damage may be the result of the cumulative effect of subconcussive blows, even in the case of concussion, which scientists have previously always viewed as an acute injury (e.g. one that results from one hard hit).

Now, a remarkable series of eight studies3-10by researchers at Purdue as part of their ongoing study of brain changes in high school football players has gone a long way to eliminating any remaining doubt that repetitive head impacts, such as sustained by players in American football, result in brain abnormalities and impaired neurocognitive functioning during a football season, and that those effects persist long after the season.

Here's what the Purdue researchers found:

the number of head impacts was related to substantive neurophysiological changes during the course of a football season, with all players sustaining more than 500 cumulative head impacts "flagged" for scoring more poorly on at least one component of the ImPACT neurocognitive test compared to their baseline, and/or displaying a statistically significant difference between pre- and post-season fMRI scans on 11 or more of 116 "regions of interest" in the brain.[3]

high magnitude hits (over 60 g's of linear force) players accrued over the course of a season were more likely to prompt abnormal biochemical/metabolic responses in regions of the brain responsible for executive and motor function, with abnormal increases in metabolites sometimes followed by metabolic decreases, all dependent on the timing, number, magnitude, and location of blows to the helmet.[4] The findings led the researchers to conclude that, with such "diverse metabolic consequences to accumulating sub-concussive blows, such competing mechanisms could (1) lead to no noticeable differences in overall metabolic levels and (2) ultimately mask symptoms in injured athletes," and provided "further evidence for a cumulative effect of head blows on neural health."

players who sustained more than 900 hits over the course of a season were much more likely than players hit less than 600 times in a season to be flagged by ImPACT, fMRI, or both.[5]

players who averaged more than 50 head impacts per week (coincidentally, the typical number of plays a high school football offense or defense ran in a game) were flagged at a rate of 83% while those who received less than 50 hits per week were only flagged 43% of the time, a threshold the researchers considered significant.[5]

when tested between 2 and 5 months after the football season ended, 6 out of 10 players had results which were flagged as abnormal, 11 by ImPACT, 12 by fMRI, with 3 flagged by both.[5] The findings led the researchers to conclude that using a neurocognitive test such as ImPACT, more commonly used by clinicians in measuring the effects of concussion and assisting in making return to play decisions, in combination with fMRI, which is sensitive to more subtle changes in brain physiology that may not be exhibited in cognitive performance, may provide a better assessments of a player's brain health than either measure by itself.

where on the helmet a player was hit most rather than the number of hits was the best predictor of changes in the brain, with hits to the facemask most predictive, suggesting that a player's style of play may be particularly important in determining brain changes resulting from subconcussive impacts.[6]

abnormal brain activation patterns while players performed tasks involving visual working memory appeared to be related to exposure to contact: after several months of play, the players exhibited a high rate of deviation from their respective pre-season measures of brain activation, with the amount of abnormal activity increasing during the primary months of contact (August-October), only beginning to drop more than two months after the season ended (October/November), and not returning to baseline again until February-April.[7] "These results," said the authors, "show that flagging is sensitive to changes in ... visual working memory tasks and could be used as a measure of a subject's neural health. ... By making use of flagging, along with other measures of neural health, it should be possible to make more informed decisions about the health of contact sports players."

cerebrovascular reactivity (CVR), a measure of the ability of blood vessels in the brain to dilate to compensate for increased levels of carbon dioxide in the blood, such as occurring during exercise, was significantly reduced in almost all football athletes during the first six weeks of the contact season.[8] While a slight majority of the players had reduced CVR when tested during the second six weeks of the season, half still had reduced CVR at MRI scanning sessions conducted 5-6 months after the end of the contact season, changes which the authors said were most likely due to contact. The findings were viewed by the researchers as demonstrating that the onset of subconcussive blows had "at least a transient effect on the brain, but also suggest that the brain can adapt to [the contact] with an eventual return to baseline." The researchers expressed concern that athletes may be at risk of incurring symptomatic injury during period their brains were trying to adapt to contact at the beginning of the season. Noting that. in most states football teams typically switch from limited contact levels prior to the season to two practices a day, at least one of which includes contact, they expressed concern that, based on their findings, "the brain may not be able to adjust quickly to this change, leaving players at increased risk for injury" at the beginning of the football season. They thus suggested that it might be better for teams to increase the amount of contact more gradually to allow players' brains to adapt so as to reduce the risk of serious injury.

(Interestingly, a number of state high school athletic associations, including Illinois, Alabama, Minnesota, and Kansas, among others, have enacted rules changes establishing such a progression up to full-contact in preseason practices. In Illinois for example, the Illinois High School Association (IHSA) limits equipment to helmets only during the first two days of practice, helmets and shoulder pads the next three days, with full pads being allowed on the sixth day of the acclimatization period. Similar progressions have also been adopted in Alabama, Minnesota and Kansas, among others.)

the number of connections in the Default Mode Network (e.g. regions of the brain that are active when a person is not focused on the outside world and the brain is at wakeful rest) were significantly reduced from pre-season levels at one month into the season, increased significantly at month two, but were still significantly reduced at 5 of 6 sessions during the post-season.[9] The authors said that the within-competition-season behavior of DMN connectivity "suggests that compensatory mechanisms in the brain may be required to maintain normal behavior, and may require several weeks of stable mechanical loading [i.e. a consistent level of contact] to achieve this goal." They sought to explain the apparently discordant observations of a decrease in DMN connectivity followed by an increase in month 2 as being due to the brain's initial inability to maintain the pre-season level of DMN connectivity after the onset of head impacts, but then its ability to adapt to a consistent level of contact during month 2. The generally decreasing trend in DMN connections during the later post-season they suggested may possibly indicate a longer-term repair process after exposure to prolonged periods of contact, with the authors hypothesizing that the brain initially adapted to the damage by finding alternate pathways for transmitting information, and then "pruned" the pathways after they were no longer needed.